Removal of acid constituents from gas mixtures



Aug. 16, 1966 B. B. WOERTZ REMOVAL OF ACID CONSTITUENTS FROM GAS MIXTURES 2 Sheets-Sheet l Flled Dec.

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M9505 ZOQQQG mibu 354090 INVENTOR.

BYRON B. WOERTZ kmuz 3 5km kqw ATTORNEY.

United States Patent 3,266,219 REMOVAL OF ACID CONSTITUENTS FROM GAS MIXTURES Byron B. Woertz, Crystal Lake, 11]., assignor, by mesne assignments, to Union Oil Company of California, Los Angeles, Calif., a corporation of California Filed Dec. 27, 1962, Ser. No. 247,699 11 Claims. (CI. 55-48) This invention relates to a selective solvent for removing acid gases from admixtures with non-acidic constituents, and more particularly, to an improved process for the removal of carbon dioxide from a gaseous mixture of hydrocarbons and/or other non-acidic constituents containing carbon dioxide by the use of a selective solvent consisting essentially of methyl dimethoxyacetate, also known as methyl glyoxylatedimethyl acetyl.

The acid gas content of natural gases varies between broad limits, depending on the field from which it was produced. Natural gases produced from some subterranean reservoirs contain undesirably high concentrations of acid gases, such as carbon dioxide and hydrogen sulfide. Before these gases can be sold, it is necessary that the high concentration of acid gases be removed or at least reduced to an acceptable concentration. Various methods of removing acid gases from natural gas have been proposed. The removal of hydrogen sulfide has been accomplished by several acceptable commercial methods, but the removal of carbon dioxide remains a problem in the art.

Since carbon dioxide is chemically reactive while the hydrocarbon gases are relatively inert, one approach taken in attempting to solve the problem of removing carbon dioxide from mixtures containing the same is the use of solvents which react with carbon dioxide to remove it in chemically combined form. For example, (1) hot potassium carbonate and (2) monoor diethanolamine have been proposed for removing carbon dioxide from natural gas. However, in order for any separation process to be practical, it must be possible to regenerate and recycle the solvent. It is apparent that regeneration of the chemically reactive solvents is expensive, and processes based on them are especially expensive when high concentrations of CO are involved.

It has also been proposed to remove carbon dioxide from gaseous mixtures by the utilization of solvents which have a selective solubility for carbon dioxide. Selective solvents currently used commercially in carbon dioxide removal processes are water, methyl alcohol, acetone and propylene carbonate. Propylene carbonate extraction is the most economical method of removing large concentrations of carbon dioxide from high pressure methane.

It is, therefore, a primary object of this invention to provide a process for removing acid gases from gaseous mixtures containing same. Another object of this invention is to provide a process for removing carbon dioxide from a hydrocarbon gas mixture containing same. Still another object of this invention is to provide a process for removing carbon dioxide from gaseous mixtures utilizing a superior selective solvent consisting essentially of methyl dimethoxyacetate. A still further object of this invention is to provide a process for removing carbon dioxide and moisture from gaseous mixtures containing the same utilizing a solvent consisting essentially of methyl 3,266,219 Patented August 16, 1966 dimethoxyacetate in combination with a second, higher boiling, hygroscopic solvent. These and further objects of this invention will become apparent and be described as the description herein proceeds and reference is made to the accompanying drawings in which:

FIGURE 1 is a diagrammatic illustration of an absorption-desorption system for carrying out the process of this invention; and

FIGURE 2 is a diagrammatic illustration of an alternative embodiment of an absorption-desorption system for carrying out the process of this invention utilizing a combination of solvents, namely, a solvent consisting essentially of methyl dimethoxyacetate and a higher boiling hygroscopic solvent.

This invention is based on the discovery that a solvent consisting essentially of methyl dimethoxyacetate, which has the following structure:

CH: O 0

is an efiective solvent for removing carbon dioxide from gaseous mixtures containing hydrocarbons and/or other non-acidic constituents. In general, this invention contemplates the removal of carbon dioxide from gaseous mixtures containing same by treatment with a selective solvent consisting essentially of methyl dimethoxyacetate. While the process of this invention is especially useful for removing carbon dioxide from natural gas, it is also applicable to the treatment of any carbon dioxide-contain ing gaseous mixture as long as the solvent has a selectivity for extracting carbon dioxide therefrom. For example, the solvent of this invention can be used for removing carbon dioxide from flue gas, hydrogen, or reformed gas for ammonia synthesis. The selective solvent of this invention is also generally efiective for removing hydrogen sulfide from gaseous mixtures.

The solvent of this invention may be used in the pure form and in admixture with inert solvents to modify one of the properties of the solvent, such as to modify its capacity and/or selectivity for absorbing carbon dioxide. The inert solvent is defined as one which is unreactive toward methyl dimethoxyacetate (and other solvent constituents) and the constituents of the gas being treated. In general, the solvent mixture may contain at least about 50% by volume of methyl dimethoxyacetate and up to about 50% by volume of at least one inert solvent. Preferred inert solvents are liquids which are also selective absorbents for carbon dioxide. Examples of suitable solvents with which the solvent of this invention may be used in admixture include propylene carbonate, ethylene carbonate, N,N-dimethylformamide, methyl acetoacetate and hydracrylonitrile. Ethylene carbonate, having a melting point of about 95 F., is considered a liquid in this specification since mixtures of it with other solvents are usually liquid at ambient temperatures, e.g., 80 F. An efiective solvent blend, for example, is one consisting of 50-80 vol. percent of methyl dimethoxyacetate and 20-50 vol. percent of methyl acetoacetate.

The suitability of methyl dimethoxyacetate as a selective solvent for carbon dioxide has been demonstrated experimentally in a series of tests wherein the capacity of a number of solvents for carbon dioxide and propane, and

the selectivity for carbon dioxide relative to propane, were determined. Solubilities were measured by injecting a measured volume of solvent into an evacuated Dumas bulb, measuring the vapor pressure of the solvent, and then metering in suflicient carbon dioxide to bring the bulb to atmospheric pressure. A series of calculations provided a corrected solubility of carbon dioxide at 80 F. The propane solubilities were similarly measured. The ratio of the carbon dioxide solubility to the propane solubility was termed the selectivity ratio. The results of this comparison are shown in Table I. In the following table the numbers in parentheses after several of the solvents refer to the US. patents in which the solvents are disclosed and claimed as acid gas solvents.

*Solubilities determined here are the volumes of gas dissolved, measured at 1 atmosphere and 80 F., per volume of solvent measured at 80 F., when the partial pressure of dry gas above the solvent is one atmosphere.

This comparison shows that while the selectivity of propylene carbonate for carbon dioxide in the presence of propane is higher than that of methyl dimethoxyacetate, methyl dimethoxyacetate has a higher capacity for carbon dioxide. It can also be seen that the solvent of this invention has a higher selectivity for carbon dioxide than most of the other solvents.

The process of this invention is carried out using conventional absorption procedures, wherein the gaseous mixture is contacted with the selective solvent of this invention in either batchwise or counter-current treatment. Successive batchwise extractions also can be used. In the preferred method of practicing the invention, the gaseous mixture to be treated is contacted in a counter-current absorption tower with the absorbent in a continuous flow method. The spent solvent is continuously withdrawn from the absorption tower and is introduced into a flash chamber and/ or air-stripping column to remove the absorbed gases. Vacuum flash can be substituted for air stripping, if desired. The regenerated solvent is then recycled through the absorption tower where it is used again.

The extraction process is preferably carried out at temperatures within the range of about to 100 F. although higher and lower temperatures may be utilized. It will be evident that the minimum temperature at which any specific solvent can be used is the minimum temperature at which the composition is a liquid. Pressures from about 100 to 11 500 p.s.i.g. (pounds per square inch gauge) may be used. The vaporization loss of the solvent is a factor to be considered in determining the contacting conditions. The maximum contact temperature should be limited to prevent an excessive loss of the solvent. In general, the feed gas and solvent are contacted at a rate of 5 to 150 gallons of solvent per M cf. of gas.

This invention is best understood by reference to FIG- URE 1, wherein a feed gaseous mixture such as natural gas, containing carbon dioxide which is to be removed therefrom, is fed through line into the bottom of absorber 12 after it passes in indirect heat exchange with the residue gas from absorber 12 in heat exchange 14 and rich absorbent in heat exchanger 16. The absorbent consisting essentially of methyl dimethoxyacetate which is hereinafter referred to merely as the absorbent, is introduced into the top of absorber 12 through line 18. Ab-

sorber 12 can be any suitable absorption column, such as a vertically extended column, containing appropriate packing or trays to assure intimate counter-current contact of the rising feed mixture with the downflowing absorbent, and cooling coils 20 to provide the desired degree of cooling, as illustrated in the drawing. Absorber 12 is maintained under such conditions of pressure, usually superatmospheric, and temperature that carbon dioxide is absorbed from the feed mixture. As previously indicated, the absorption process is preferably carried out at a temperature within the range of about 0 to F. and a pressure between about 100 and 1500 p.s.i.g. The amount and rate of carbon dioxide absorption increase directly with an increase in the pressure maintained in the absorption zone, in the lower pressure range. The feed mixture, from which at least part of the carbon dioxide content has been absorbed, is then removed from absorber 12 through line 22, passed in indirect heat exchange with the feed gas in exchanger 14, and fed to a suitable receiver or otherwise disposed of.

The rich absorbent, containing absorbed carbon dioxide, is withdrawn from absorber 12 through line 24. Then, the rich absorbent undergoes a controlled pressure reduction, such as by being passed successively through expansion valve 26 and heat exchanger 16 into flash chamber 28. Flash chamber 28 is maintained at a pressure below the pressure of absorption column 12, but above atmospheric pressure, e.g., about 50 to 400 p.s.i.g. so that the flash gas is about 5 to 10% of the inlet gas in line 10 at standard temperature and pressure (14.7 p.s.i.a. and 60 B). As the absorbent undergoes pressure reduction, it becomes cooled due to the-loss of heat of absorption acquired in absorber 12 and expansion of absorbed carbon dioxide to a lower partial pressure. Part of the absorbed carbon dioxide and non-acidic constituents of the feed gas are withdrawn from flash chamber 28 through line 30. The gases in line 30 are compressed in compressor 32 and passed through line 34 to be mixed with the feed gas before it passes through exchanger 14. The partially desorbed absorbent is withdrawn from flash chamber 28 through line 36 and fed into stripping column 38 after it has passed successively through expansion valve 39 and exchanger 40, in indirect heat exchanger with the lean absorbent withdrawn from column 38.

Air or other inert stripping gas is introduced into stripping column 38 through line 42. Stripping column 38 is provided with heating coil 44 to apply heat if necessary.

'In stripping column 38, substantially all of the remaining absorbed carbon dioxide is removed from the absorbent and withdrawn through line 46 to be disposed of as desired. Line 46 may be provided with a vacuum pump, if desired. The resulting lean absorbent is then withdrawn from stripping column 38 through line 48, where it is forced by pump 50 through heat exchanger 40 and then returned to absorber 12 through line 18. It will be evident that line 18 may include a second heat exchanger, not shown, to further cool the lean absorbent after it has passed through heat exchanger 40.

Other alternative absorption-desorption processes will be apparent to those skilled in the art. For example if it is not desired to limit the loss of the non-acidic constituents of the feed gas, the rich absorbent may be desorbed in a flash or stripping step and the off-gases disposed of as desired. The partially desorbed absorbent may then be recycled to the absorber or introduced into a stripping column to remove the last vestages of absorbed carbon dioxide before it is recycled to the absorption column. The residue gas from the absorber and/or the flash gas may be passed through a solid bed of an absorbent, such as silica gel, activated alumina, activated carbon or a synthetic zeolite, to separate vaporized absorbent therefrom.

In an alternative embodiment of this invention, the absorbent of this invention may be used in combination with a second higher boiling hygroscopic solvent which is absorptive of it, to avoid the uneconomically high solvent losses which are common in conventional processes. The hygroscopic solvent serves to remove moisture from the feed gaseous mixture, as well as to recover the absorbent which would otherwise be lost in the product gas stream. The hygroscopic solvent is any of the polyhydric alcohols or glycols which have been proposed or used for moisture extraction, such as ethylene glycol, propylene glycol, diethylene glycol, triethylene glycol and aqueous solutions thereof. However, the glycol selected shouldordinarily be higher boiling than the selective solvent, in this case methyl dimethoxyacetate.

This alternative embodiment is best understood by reference to FIGURE 2 wherein the numeral 110 represents the line through which the gas to be treated, for example, a natural gas consisting of hydrocarbons, carbon dioxide, and moisture, is fed into the bottom of contactor 112, in which it is countercurrently contacted with a mixture of the methyl dimethoxyacetate and hygroscopic solvents entering through line 114. Contactor 112, which may contain any suitable type and arrangement of trays or baflles as required for intimacy of contact between the absorbent mixture and feed gas, is preferably maintained at a temperature within the range of about 20 to 100 F. and pressure within the range of about 100 to 1500 p.s.i.g., although higher and lower temperatures and pressures may be used. A suitable example of the absorbent mixture entering contactor 112 through line 114 is 75 to 95% by weight of a glycol, such as triethylene glycol, O to 5% by weight of methyl dimethoxyacetate and 0 to 20% by weight of water, the amount of water being indirectly proportional to the contacting temperature to reduce the glycol viscosity. For low temperature operation, as at about 0 to 40 F. for example, a small amount of water is required in the glycol to maintain a satisfactory viscosity; the glycol containing a minor amount of water will still dehydrate the feed gas satisfactorily. In contactor 112, much of the water content of the feed gas stream is taken up by the glycol solvent. Glycol absorbent, containing water and some methyl dimethoxyacetate absorbent, is withdrawn from contactor 112 through line 116. The resulting natural gas methyl dimethoxyacetate absorbent mixture leaves contactor 112 through line 118, and flows to chiller-condenser 120, in which its temperature is reduced to absorption temperature, for example, 0 to 80 F. The condensate is withdrawn from chiller-condenser 120 through line 122 and may be combined with the liquid in line 116. The condensate in line 122 is preferably first introduced into fractionating unit 123 to separate condensed hydrocarbon before it is combined with the liquid in line 116 by way of line 124.

The chilled, uncondensed components of the natural gas-methyl dimethoxyacetate absorbent mixture pass from chiller-condenser 120 through line 125 to the absorbing zone which, as illustrated, may be contained within the lower and upper sections 126 and 128 of a single vertically extended absorption tower 130, the two sections being separated by liquid trap-out tray 132. Absorber 130 can have any suitable arrangement of packing or trays to assure intimate countercurrent contact of the rising feed mixture with the d-ownflowing absorbent. In section 126, the gas is countercurrently contacted with methyl dimethoxyacetate absorbent entering through line 134, which removes at least part of the acid gas constituents. Rich methyl dimethoxyacetate absorbent, containing absorbed carbon dioxide, is withdrawn from absorption tower 130 through line 136. The scrubbed gas passes on upward in tower 130 past trap-out tray 132 into upper section 128, where it is countercurrently contacted with the triethylene glycol entering through line 138. The triethylene glycol absorbent scrubs any vaporized methyl dimethoxyacetate absorbent from the natural gas, and the scrubbed natural gas product, of reduced carbon dioxide content, is withdrawn through line to a suitable receiver or other disposition. The mixture of triethylene glycol and methyl dimethoxyacetate absorbents from upper section 128 is trapped on tray 132, and is withdrawn through line 142 and introduced into contactor 112 through line 114.

The methyl dimethoxyacetate absorbent in line 136 enters a combined flashing and contacting zone which, as illustrated, may be lower flashing zone 144 and upper contacting zone 146 in a single vertically extended vessel 148, with the upper and lower sections being separated by liquid trap-out tray 150. The rich methyl dimethoxyacetate absorbent enters flash and stripping zone 144, which is maintained at a pressure below the absorption column pressure, and usually at about atmospheric pressure, where it undergoes pressure reduction to flash-oft absorbed carbon dioxide. Zone 144 is provided with suitable contacting devices such as trays or packing. An inert stripping gas, such as air, may be introduced into zone 144 through line 145. The lean methyl dimethoxyacetate absorbent is withdrawn from flash zone 144 through line 152 and is introduced back to absorption tower 130 by pump 154 through line 134. Carbon dioxide containing entrained or vaporized methyl dimethoxyacetate absorbent leaves flash zone 144, and passes through trap-out tray into contacting zone 146 where it is countercurrently contacted with glycol solvent entering through line 156. The triethylene glycol scrubs any vaporized methyl dimethoxyacetate absorbent from the carbon dioxide, and the carbon dioxide and stripping gas are withdrawn through line 158 to be disposed of as desired.

The resulting mixture of triethylene glycol and methyl dimethoxyacetate absorbents from contacting zone 146 of vessel 148 is trapped on tray 150 and withdrawn through line 160. Part of the solvent mixture in line 160 is passed by pump 162 to branch line 138 from where it is introduced into upper section 128 of contactor 130. The remainder of the solvent in line 160 is passed through line 164 and combined. with the solvent mixture in line 142 to form the mixture entering contactor 112 through line 114.

The triethylene glycol containing water and some methyl dimethoxyacetate absorbent, which was withdrawn from contactor 112 through line 116, is 'fed into glycol still 166, equipped with reboiler 168. Regenerated triethylene glycol, substantially free of methyl dimethoxyacetate absorbent and water, is removed from reboiler 168 through line 169 and forced by p ump 170 through heat exchanger 172, where it passes in indirect heat exchange with the mixture in line 116. From heat exchanger 172, the regenerated triethylene glycol passes through line 174, cooler 176 and line 156 to contacting zone 146 of Vessel 148. If desired, part of the mixture in line 116 may be made to by-pass still 166 by opening valve 178 so that it passes through line 180 into line 174. Water and methyl dimethoxyacetate absorbent are withdrawn as overhead from still 166 through line 184, and are passed through overhead condenser 186 and into reflux drum 188. Non-condensable hydrocarbon gases are vented from reflux drum 188 by line 190. From reflux drum 188, part of the liquid is returned through line 192 to still 166, while the balance is passed through line 194 to still 196.

Still 196 is provided with reboiler 198 from which regenerated methyl dimethoxyacetate absorbent is withdrawn through line 200 and passed in heat exchange with liquid in line 194 in heat exchanger 202. The methyl dimethoxyacetate is then withdrawn from heat exchanger 202 through line 204 and forced by pump 206 through cooler 208 and line 210 into flash zone 144 of vessel 148. The overhead from still 196, consisting mainly of water, is withdrawn through line 212 and passed through condenser 214 into reflux drum 216, from which hydro- 7 carbons are withdrawn through line 213. Part of the water in reflux drum 216 is returned through line 229 to still 196, and the remaining portion is discarded through line 222 or fed through line 224 into line 156 by opening 8 of comparison, Tables V and VI give the composition of the process streams wherein the ga is contacted wtih propylene carbonate.

The e'ificiency of an absorbent containing at least 50 valve vol. percent of methyl dimethoxyacetate as a selective Example I solvent for the removal of acid gases from a gaseous mix- Tables II to VI are illustrative of the process streams ture containing the same can be seen by a review of the of the process depicted in FIGURE 1 wherein a natural data in the tables Tables 11, III and V show that under 2 mixture is contacted in absorber Containing ten similar conditions methyl dimethoxyacetate, including a theoretical trays, With the lean absorbfintone hundred 50/ 5O blend thereof with methylacetoacetate, has a better pound 1nols of the natural gas per unit time are contacted capacity f carbon dioxide than propylene carbonate in different runs with the absorbent in such amounts that (note the respective Circulation rates), although the Se1ec h Fesldue gas Fontams about VOL Percent of Carbgn tivity of the solvents of this invention are slightly lower diornde. The rich absorbent withdrawn from absorber 15 than the sdectivl-ty of propylena Carbonate (note the 12 1s lsothermauy flashed m fl chamber 28.?nd the drocarbon content of the residue gas). Again comparing pressure of the partially desoibed absorbent withdrawn from flash chamber 23 is reduced to atmospheric pressure the fificecnvngss of the Solvent blend (Table wlth in stripping column 38. Tables 11 to IV give the compo- Prom/lens 9919093156 (T9171? VI) at 700 absorptlon: the sitions of the process streams where the gas is contacted 2O blend of thls Invention has lmpfoled p y With a little with the absorbent of this invention and, for the purpose reduction in selectivity.

TABLE II Absorber Maintained at 0 F. and 600 p.s.i.a.

Absorbent Circulation Rate of 17.2 GaL/M 0.1. 0! Feed Gas Rich Absorbent Flashed at 0 F. and 150 p.s.1.a.

Residue Gas Mols of Mols of (Line 22) Mols of M 01s of Atmos Percent Loss Component Inlet Gas Rich Solvent Flash Gas Strip Gas.

(Line 10) (Line 24) (Line (Line 46) Mols M01 Pcrccnt Methane 72. 0 71. 49 10. 63 3. 39 2. 87 0.51 0. 71 Ethane 6. 0 5. 08 G. 44 1. 66 O. 73 O. 92 15. 3 Propane 2. 0 0. 74 0. 94 1. 56 0. 30 1. 26 G3. 0 CO 19. 0 1. 57 1. 99 20. 46 3. 03 17. 43 91. 7 n-Butane 1. 0 0 0 1. 08 0. 08 1. 00 100. 0 Absorbent 0 0 0 45. 00 0 0 Total 100.00 78. 88 100. 00 73. 15 7. 01

TABLE III Solvent: 50 vol. percent Methyl Dimethoxyacetate and 50 vol. percent Methyl Acetoacetatc Absorber Maintained at 0 F. and 600 p.s.i.a. Absorbent Circulation Rate of 23.0 Gal./M 0.1. of Feed Gas Rich Absorbent Flashed at 0 F. and 150 p.s.i.a.

Residue Gas Mols of Mols of (Line 22) Mols of Mols of Atmos. Component Inlet Gas Rich Solvent Flash Gas Strip Gas Percent Loss (Line 10) (Line 24) (Line 30) (Line 46) Mols M01 Percent TABLE IV Solvent: 50 vol. percent Methyl Diinethoxyacetate and 50 vol. percent Methyl Acetoacctate Absorber Maintained at "F. and 600 p.s.i.a.

Absorbent Circulation Rate of 54.1 Gal./M 0.1. of Feed Gas Rich Absorbent Flashed at 70 F. and 200 p.s.i.a.

Residue Gas Mols of Mols of (Line 22) Mols of Mols of Atmos. Component Inlet Gas Rich Solvent Flash Gas Strip Gas Percent Loss (Line 10) (Line 24) (Line 30) (Line 46) Mols M01 Percent Methane 72. 0 70. 57 91. 19 6. 61 5. 18 1. 43 2, 0 Ethane- 6. 0 4. 71 6. 09 2. 39 1.11 1. 29 21. 5 Propane 2. 0 0. 56 0. 72 1. 94 0. 50 1. 44 72. 0 CO 19. 0 1. 55 2.00 21. 94 4. 49 17. 45 91. 8 n-Butane 1. 0 0 1. 16 0. 16 1.00 100. 0 Absorbent 0 0 0 150. 74 0 O Total 100.00 77. 39 100. 00 184. 78 11.44 22. 61

TABLE V Solvent: Propylene Carbonate Absorber Maintained at T. and 600 p.s.i.a. Absorbent Circulation Rate of 26.2 GaL/M c.f. of Feed Gas Rich Absorbent Flashed at 0 F. and 150 p.s.i.a.

Residue Gas Mols oi Mols of (Line 22) Mols of Mols of Atrnoe. Component Inlet Gas Rich Solvent Flash Gas Strip Gas Percent Loss (Line 10) (Line 24) (Line 30) (Line 46) Mols Mol Percent Total 100. 00 80. 10 100. 00 123. 33 5. 00 19. 90

TABLE VI Solvent: Propylene Carbonate Absorber Maintained at 70 F. and 600 p.s.i.a. Absorbent Circulation Rate of 66.9 GaL/M c.i. of Feed Gas Rich Absorbent Flashed at 70 F. and 150 p.s.i.a.

Residue Gas Mols oi Mols of (Line 22) Mols of Mols of Atrnos. Component Inlet Gas Rich Solvent Flash Gas Strip Gas Percent Loss (Line 10) (Line 24) (Line (Line 46) t Mols Mol Percent Methane 72. 0 71. 33 89. 69 4. 92 4. 24 0. 67 0. 93

6. 0 5. 36 6. 74 1. 63 0t 99 0. 64 10. 7 2.0 1. l9 1; 49 1.32 0.51 0.81 40. 5 19.0 1. 59 2.00 24.11 6. 74 17. 37 91 4 1. 0 0. 06 O. 08 1. 20 0.27 0. 93 93. 0 0 0 0 253 24 O 0 The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The process of removing gaseous acid gas from admixture with gaseous C -C alkane hydrocarbons, which comprises contacting said gaseous admixture with an acid gas selected from the group consisting of carbon dioxide and hydrogen sulfide solvent consisting essentially of methyl dimethoxyacetate, under conditions resulting in selective absorption of said acid gas, and separating the unabsorbed components of said gaseous admixture from said solvent.

2. The process according to claim 1 in which said acid gas is carbon dioxide.

3. The process according to claim 2 in which said gaseous admixture is a natural gas and said solvent additionally contains a liquid selected from the group consisting of propylene carbonate, ethylene carbonate, hydracrylonitrile, .and N,N-dimethylformamide.

4. The process according to claim 3 in which said absorbent consists of at least 50 vol. percent of methyl dimethoxyacetate.

5. The process according to claim 2 in which said gaseous admixture is passed through an absorption zone at superatmospheric pressure in countercurrent contact with said solvent and a stream of rich solvent is removed from said absorption zone and the pressure thereof reduced to flash oif carbon dioxide and at least part of the residual carbon dioxide is removed from said solvent stream following said pressure reduction and the desorbed solvent is recycled to said absorption zone.

6. The process according to claim 5 in which said contacting is carried out at a temperature from about 0 to 100 F. and a pressure from 100 to 1500 p.s.i.g.

7. The process of removing gaseous acid gas selected from the group consisting of carbon dioxide and hydrogen sulfide from admixture with gaseous C -C alkane hydrocarbons, which comprises contacting said gaseous admixture in a first contacting zone with a mixture of an acid gas absorbent consisting essentially of methyl dimethoxyacetate and a second higher boiling solvent adapted to absorb said first solvent; separately withdrawing a liquid stream consisting essentially of said second solvent and a gaseous stream comprising vaporized methyl dimethoxyacetate and unabsorbed components of said gaseous admixture fed to said first contacting zone from said first cont-acting zone; passing said gaseous stream through a cooling zone whereby part of said gaseous stream is condensed; separately withdrawing condensate and uncondensed components of said gaseous stream from said first cooling zone; combining said condensate with said liquid stream from said first contacting zone, and separating absorbed second solvent from the combined solution; contacting the uncondensed components of said gaseous stream in a second contacting zone with said first solvent, under conditions resulting in partial vaporization of said first solvent and absorption of said acid gas in the unvaporized first solvent, separately withdrawing a liquid stream of spent first solvent and a gaseous stream from said second contacting zone; contacting the gaseous stream from said second contacting zone in a third contacting zone with said second solvent, under conditions resulting in absorption of vaporized first solvent; separately withdrawing from said third contacting zone the unabsorbed components of the gaseous stream from said second contacting zone and said second solvent containing absorbed first solvent; recycling said second solvent from said third contacting zone to said first contacting zone; and separating the absorbed acid gas from said spent first solvent.

8. The process according to claim 7 in which the uncondensed components of said gaseous stream are passed through said second contacting zone at superatmospheric pressure in countercurrent contact with said first solvent the pressure of said spend first solvent is reduced in a desorbing zone to flash off acid gas, and the acid gas separated from said spent first solvent in said desorbing zone is contacted with said second solvent in a fourth contacting zone whereby entrained vaporized first solvent is absorbed and said second solvent withdrawn from said first contacting zone is introduced into said fourth contacting zone after at least part of the absorbed first solvent is removed therefrom.

9. The process according to claim 8 in which a stream of said second solvent is removed from said fourth contacting zone and parts thereof are introduced into said first and third contacting zones.

10. The process accordingto claim 9 in which said second solvent is a glycol and said solvent contains a liquid .selected from the group consisting of propylene carbonate, ethylene carbonate, hydracrylonitrile, and N, N-dimethylformamide.

11. The process of removing a gaseous acid gas selected from the group consisting of carbon dioxide and hydrogen sulfide from admixture With gaseous C -C 'alkane hydrocarbons, which comprises contacting said gaseous admixture with about 5 to 150 gallons of methyl dimethoxyacetate per M.c.f. of the gaseous mixture measured at 14.7 p.s.i.a. and 60 F., at a temperature of about 0 to 100 F. and a pressure of about 100 to 1500 p.s.i.g., and separating the unabsorbed components of said gaseous admixture from said methyl dimethoxyacetate.

References Cited by the Examiner UNITED STATES PATENTS 2,139,375 12/1938 Millar et a1. 55-73 2,781,863 2/1957 Bloch et a1 55-73 X 2,863,527 12/1958 Herbert et 211.

2,926,751 3/1960 Kohl et a1. 5568 X 3,097,917 7/1963 Dotts et a1. 5573 X FOREIGN PATENTS 596,862 2/1957 Canada. 596,693 2/1957 Canada. 728,444 4/ 1955 Great Britain. 750,399 6/ 1956 Great Britain.

REUBEN FRIEDMAN, Primary Examiner. D. TALBERT, Assistant Examiner. 

1. THE PROCESS OF REMOVING GASEOUS ACID GAS FROM ADMIXTURE WITH GASEOUS C1-C3 ALKANE HYDROCARBONS, WHICH COMPRISES CONTACTING SAID GASEOUS ADMIXTURE WITH AN ACID GAS SELECTED FROM THE GROUP CONSISTING OF CARBON DIOXIDE AND HYDROCARBON SULFIDE SOLVENT CONSISTING ESSENTIALLY OF METHYL DIMETHOXYACETATE, UNDER CONDITIONS RE- 